This invention relates to an ignition system of the capacitor discharge type for internal combustion engines.
In a conventional ignition system, two coils for generating electric current or voltage (hereinafter referred to as "generating coils") are provided in a magneto electric generator which rotates in synchronization with an internal combustion engine, and the capacitor is charged by a voltage induced in one of the two generating coil, and the capacitor thus charged is discharged through the primary winding of an ignition coil when a thyristor is made conductive by a signal voltage which is produced, in synchronization with the ignition timing, by the other generating coil.
However, this conventional ignition system involves the following problems because of the two generating coils. That is, troubles, such as disconnection or breaking of coils, originated from the coils are frequently caused, the dimensions of the generator is inevitably increased greater as much as the volume occupied by the two generator-coils, and the circuit of the ignition system is liable to become intricate.
In general, internal combustion engines whose load is extremely variable are accompanied by excessive rate of rotation whenever its load is removed. For instance, an internal combustion engine for chain saws will rotate at an excessive speed when it has completely sawed through a log because at that moment the internal combustion engine abruptly loses its load. This condition will be caused also in an internal combustion engine of an outboard motor when it jumps. This is undesirable for the operation of internal combustion engines, and several methods such as methods of controlling a throttle valve or ignition have been tried to prevent the trouble.
Furthermore, in small engines (in general, started manually) such as a small all-purpose engine which requires no battery, an ignition system cooperated with a magneto electric generator is employed and the ignition system is preferable to make contact-less by using a semiconductor switch in charging and discharging a capacitor because of its maintenance free. In this case, frequently, a timing detector, an electronic circuit, generating coil and a high tension coil of the ignition system are divided into a few units which are mutually connected by wires. Therefore, this contact-less ignition system is more complicated in handling than a conventional ignition system in which contacts are used for ignition.
In addition, in the conventional ignition system employing a thyristor as its switching element for discharging the capacitor, a voltage between the cathode and the control electrode of the thyristor which is necessary for rendering the thyristor conductive varies with its ambient temperature, as a result of which the ignition timing of the internal combustion engine provided with this ignition system is also varied. Since the ambient temperature is greatly changed during the start period of the engine, the voltage variation due to the variation of the ambient temperature is too great to be neglected.